Location of O2 Sensor

[Molson02536]

Molson,

The other day - Thursday, I pulled a code and replaced the vapor canister. It took until today to get the part in. We now have a new canister in as well.

The truck is running bad - very bad. The fastest it will go is 20 mph.

Now, we just pulled the codes again and here's what I have:

Code 24 - Vechcile speed sensor

and

Code 42 - EST (Electronic Spark Timing)

At this point, I have no idea what to replace next and/or what to try. It was running like a brand new truck for two days.

Just to give you a bit more information, a few weeks ago we put throttle body cleaner and put Chemtrol B-12 in the gas and, as I said, it ran great but, on the way back home from working (and hauling a load) is when it started running bad and we ordered the canister.

Thank you for all your help - it is much appreciated. Tony

Makes me want to buy your Sonoma

The speed sensor in located on the transmission's tail shaft on the drivers side, common for the sensor and or the wire leads to go bad after a few years and i do believe you have more years then most cars last for. Inspect the lead wires and also you may want to replace the fuel filter again, running fuel injector cleaner through the fuel system may have cleaned out your gas tank and fuel lines which the fuel filter would have caught.

As for your code 42, inspect the wire leads again after you resolve your transmission issue and fuel filter. The EST may have been the result of your code 24 and your engine over revving and the spark is jumping in the distributor from being over advance from the weights in the distributor.

Keep me posted, be glad to help you as much as i can without getting my hands dirty

Circuit Description

When the system is running on the Ignition Module, that is, no voltage on the by-pass line, the Ignition Module grounds the EST signal. The ECM expects to see no voltage on the EST line during this condition. If it sees a voltage, it sets Code 42 and will not go into the EST mode.

When the rpm for EST is reached (about 400 rpm), and by-pass voltage applied, the EST should no longer be grounded in the ignition module so the EST voltage should be varying.

If the by-pass line is open or grounded, the ignition module will not switch to EST mode, so the EST voltage will be low and Code 42 will be set.

If the EST line is grounded, the ignition module will switch to EST, but because the line is grounded there will be no EST signal. A Code 42 will be set.

Problem Description:

Code 42 is set when:

EST line open or grounded.
By-pass line open or grounded.

Check your computer grounds and your engine grounds. Reach down behind distributor and wiggle the four wire connector that plugs in the ignition module. If the engine stumbles, there's the problem. If not take out the module and have it tested at Autozone or other place, they have a test for the ESC. Make sure it's not a bad wire on the pickup coil in the distributor.

NO worries, your not alone in this type of problems.

 

[djk4013]

Gmc....

Molson,

I'm a bit confused sorry, do the codes mean transmission problems now too?


After putting the canister in, and starting the truck up, before I even moved the truck seemed to be fouling out as if I wasn't getting good response to the pedal or something?

Do you think this could be the cadalytic converter? Standing in the back of the truck it doesn't sound like it's exahusting right.....

 

[Molson02536]

The following understanding of how the electronic spark timing (EST) system works - what is taking place and why - is a big step toward a quicker and easier diagnosis of this circuit. When the engine is turning over, but below the run threshold of 400 RPM, the ECM holds the module's bypass voltage too low (0 volts) to energize its solid-state switch (ignition module). Another way of looking at the solid-state switch is to think of it as a relay. When voltage is applied, the relay is energized. With no voltage applied, the relay is de-energized. The pick-up coil/crank sensor pulses are amplified and shaped by the ignition module. These pulses are used to complete the circuit creating a magnetic field in the ignition coils primary winding that, when collapsing, will induce a high voltage in the secondary winding. Therefore, for every crank sensor/pick-up pulse, the coil is triggered. This is known as the bypass mode or module mode of operation. In this mode, the engine is running on the timing advance that is built into the ignition module.

With the voltage low on the bypass wire, the EST is pulled to ground through a resistor in the ignition module. This keeps the voltage on the EST wire at around 300 mV. When the ECM sees the RPM over the run threshold, it will then apply 5 volts to the bypass wire that will activate the solid-state switch (relay). This in turn will switch the EST from ground to the base of the transistor that controls the primary coil. In this mode, the primary coil winding is being triggered by the altered signal sent out from the ECM. The ECM will alter the signal to the ignition module and control the timing based on the inputs from various sensors. This is referred to as the EST mode.

This is how the system is designed to work! Now let's look at some of the things that can cause problems and set a Code 42. Under 400 RPM, no voltage on the bypass wire, the ECM expects to see low voltage (300 mV) on the EST line during this condition. If it sees 0 volts, indicating an open in the EST circuit - or higher than 500 mV - it sets a Code 42 and stays in the bypass mode.

If the bypass line is open, or grounded, the ignition module will not switch to the EST mode. The ignition module needs the bypass voltage to activate the solid-state switch so the EST voltage will be low over 400 RPM and a Code 42 will set. If the EST line is grounded, the ignition module will switch to the EST - but because the line is grounded, there will be no EST signal. A Code 42 will set.

 

[Molson02536]

Molson,

I'm a bit confused sorry, do the codes mean transmission problems now too?


After putting the canister in, and starting the truck up, before I even moved the truck seemed to be fouling out as if I wasn't getting good response to the pedal or something?

Do you think this could be the cadalytic converter? Standing in the back of the truck it doesn't sound like it's exahusting right.....

A bad Catalytic converter can cause this also, Inspect the Catalytic and see what kind of exhaust flow your getting from the tail pipe. The speed sensor is a simple fix, as mentioned earlier. Inspect the wire leads on the tail shaft on the transmission.

Code 24 - Vechcile speed sensor:

Circuit Description

The ECM applies and monitors 12 volts on CKT 437. CKT 437 connects to the Vehicle Speed Sensor which alternately grounds CKT 437 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and the ECM will calculate vehicle speed based on the time between "pulses".

Problem Description:
Code 24 will set if vehicle speed equals 0 mph when:

Engine speed is between 2000 and 4400 rpm.
TPS is less than 2% (closed throttle).
Low load condition (High vacuum).
Not in Park or Neutral.
All conditions met for 5 seconds.

My Haynes manual for '82-'93 states.."The Vehicle Speed Sensor (VSS) is located next to the speedometer cable fitting on the back of the instrument cluster on early models and mounted in the transmission on later models. If it's on the back of the speedometer, it's screwed on the back and looks like a little light. Or on top of your transmission of the driver's side. Look for a square plastic plug that has four wires going into it. My experience is based on working on first gens so, yours may be different. Also, on a first gen with the speedometer mounted VSS, the VSS signal goes through an amplifer box by the fuse panel under the dash. If the amplifer box is bad or disconnected, the same code/symptoms will happen.

 

[djk4013]

Gmc.......

Molson.....I'm really thinking maybe I should replace the catalytic converter.

There is no exhaust flow coming out the tail pipes - very very little. And, it sounds baffled.

 

[Molson02536]

Molson.....I'm really thinking maybe I should replace the catalytic converter.

There is no exhaust flow coming out the tail pipes - very very little. And, it sounds baffled.

Got my fingers crossed for you. You can pick up a universal high flow Catalytic converter from Magnaflow for around $75.00 i believe at most auto parts store like Auto Zone and Pep Boys.

If it's is a plugged Catalytic converter, take note that your exhaust side will be running hot and may burn a hole in your exhaust valves. Try not to run the engine too long to ensure not to hurt the heads. Keep me posted.

http://www.magnaflow.com/